Method of making semiconductor casing

ABSTRACT

A casing for an electrical component comprises a metal lead frame having the electrical component connected thereto. A metal base member is bonded to one surface of the lead frame. A plastic housing member is bonded to a second surface of the lead frame. An adhesive seals and bonds the metal base member and plastic housing member to the lead frame so as to enclose the electrical component.

This application is a division of application Ser. No. 398,497, filedJuly 15, 1982, now U.S. Pat. No. 4,480,262.

The present invention is directed to a method and apparatus for encasinga semiconductor and more particularly for providing a highly reliablecasing with a metal substrate adhesively bonded to a preformed plastictop.

In the electronic industry, common plastic semiconductor packages areplastic dual-in-line packages (DIP), plastic Quad Paks (plastic chipcarriers) and a somewhat modified form for discrete semiconductordevices. These different configurations are basically plastic moldedpackages in which an electronic device is supported on a lead frame andmolded into or encapsulated in plastic. The resulting plastic packagesare prone to problems which can either decrease their reliability ortheir cost of manufacture.

Firstly, failures in the plastic to metal bond, i.e. between the leadframe and the plastic package, provide an avenue through which moistureand other contaminants can reach the electrical device and causecorrosion failure. This problem has been more fully explained anddocumented in an article entitled "Factors Governing AluminumInterconnection Corrosion in Plastic Encapsulated MicroelectronicDevices" by Neighbour and White, published in Microelectronics andReliability, by Pergamon Press in Great Britain, Volume 16, 1977, pages161-164.

Another source of failure may arise from the diffusion of contaminantsthrough the plastic or perhaps as a result of gradual degradation of theplastic. This type of failure has been described in an article entitled"Factors Contributing to the Corrosion of the Aluminum Metal onSemiconductor Devices Packaged in Plastics" by Olberg and Bozarth,published in Microelectronics and Reliability, by Pergamon Press inGreat Britain, Volume 15, 1976, pages 601-611.

Generally, the power consumption of the electronic components is quitemodest and the heat generated during their operation is comparativelysmall. However, as the number of functions incorporated on a single chipbecome greater and/or the functions upon the chip are spaced moreclosely together, the heat generated becomes a significantconsideration. Also, the current trend towards incorporating relatively"high power" components into integrated circuits so as to reduceexternal components for "power functions" increases the generation ofheat. Accordingly, the capability of dissipating the greater heat beinggenerated becomes more important.

The present conventional packaging technology relies on heat generatedby the chip to be transferred through the plastic to the leads. It thenpasses along the leads to the outside of the package. Ultimately, it istransferred from the leads through radiation and/or convection to theatmosphere or to the printed circuit board to which the lead frame ismounted. The heat transfer along the lead frame may be constricted bythe small dimension of the individual leads. A secondary heat transferpath is from the chip through the plastic to the outside surface of theplastic package. The heat transfer, in this case, is inhibited by theconsiderable distance which the heat must flow in order to reach thelead frame or the outside surface of the package.

In the manufacture of conventional plastic packages, the plasticencapsulating material is prevented from flowing into unwanted areas byproviding "dams" on the lead frame. This requires an extra step ofremoving the dams after curing of the material. Even with the dams, someplastic flash extrudes beyond the boundaries of the mold cavity, adheresto the lead frame and must be finally removed. Preventing and/oreliminating this flash material is an added expense and bother in themanufacture of the plastic packages.

Reliability problems are also created by the plastic encapsulant flowingaround the device, the lead frame and the lead wires. As the viscousmolding compound flows around the wires, significant dynamic stressesmay be imposed upon the wire and upon the relatively delicate bondsbetween the wire and the device and/or the lead frame. Further,materials may be present in the plastic encapsulant which may bedeleterious to the device, the bonding material or the lead frame andmay react with any of these components and particularly with the chipcausing degradation thereof.

U.S. patent application Ser. No. 390,081 entitled "Semiconductor Casing"and U.S. patent application Ser. No. 390,095 (now U.S. Pat. No.4,410,927) entitled "Improved Semiconductor Package" both by Sheldon H.Butt are directed to improvements in semiconductor packaging.

It is a problem underlying the present invention to provide a packagefor an electrical component which has a high reliability at a cost thatmay be less than the present state of the art technology.

It is an advantage of the present invention to provide a casing for anelectrical component which obviates one or more of the limitations anddisadvantages of the described prior arrangements.

It is a further advantage of the present invention to provide a casingfor an electrical component which is substantially resistant to thediffusion of contaminants.

It is a further advantage of the present invention to provide a casingfor an electrical component which provides relatively good heat transfercharacteristics.

It is a yet further advantage of the present invention to provide acasing for an electrical component which is relatively inexpensive tomanufacture.

It is a still yet further advantage of the present invention to providea casing for an electrical component which may be assembled without theuse of high temperature processes.

Accordingly, there has been provided a casing for an electricalcomponent comprising a metal lead frame having the electrical componentconnected thereto. A metal base member is bonded to one surface of thelead frame. A plastic housing member is bonded to a second surface ofthe lead frame. An adhesive seals and bonds the metal base member andplastic housing member to the lead frame so as to enclose the electricalcomponent.

The invention and further developments of the invention are nowelucidated by means of the preferred embodiments shown in the drawings.

FIG. 1 is a cross section of a typical prior art casing for anelectrical component.

FIG. 2 is a top view of a typical prior art lead frame.

FIG. 3 is a cross section of a casing for an electrical component inaccordance with the present invention.

FIG. 4 is a cross section of a casing positioned upon a printed circuitboard in accordance with the present invention.

FIG. 5 is a cross section of a molded lid having a contaminant absorbingmaterial.

FIG. 6 is a cross section of a lid having a metal foil insert.

FIG. 7 is a cross section of a plastic lid having a metal foil coveringa portion of the external surface of the lid.

A typical plastic package 10 for sealing an electrical component inaccordance with the present technology is shown in FIG. 1. This generalconfiguration is used in plastic dual-in-line packages, plastic QuadPaks and in a somewhat modified form as packages for discretesemiconductor devices. The method of producing such a plastic package isdescribed herein in order to have a better appreciation for the advancesof the present invention.

Beginning with a lead frame 12, as shown in FIG. 2, the semiconductorchip 14 is bonded to the pad 16 of the lead frame. This bonding may beaccomplished by either using a conductive adhesive as is often the casein integrated circuits or a metallic solder or brazing material as inthe case of many discrete devices. Typically, the plastic adhesivesinclude epoxies and polyimides. They may be formulated with powderedmetals, such as silver, to improve their thermal conductivity and/or toprovide moderate electrical conductivity. The metal joining materialsmay include a gold-silicon eutectic brazing compound. The bond is formedby gold plating the reverse side of the silicon chip and subsequentlyusing a diffusion anneal to achieve the desired gold-silicon eutecticcomposition. In addition, other solders may be used as required.

The interconnection between the chip 14 and the lead frame 12 istypically provided by means of small diameter lead wires 18 (usuallygold) which are metallurgically bonded to the tips of the leads in thelead frame and to the chip. In most integrated circuits, the bondingtechnique is either thermosonic or thermocompression bonding.Thermosonic bonding is a combination of mechanically inducedmetallurgical bonding at temperatures above ambient in conjunction withthe addition of ultrasonic energy. Thermocompression bonding ismechanical deformation at elevated temperature without the addition ofthe ultrasonic energy. A foil "spider" is often used as an alternativeto the wires. The foil itself may be bonded to the chip by eitherthermosonic or thermocompression bonding.

After the electrical interconnections have been made, the device isencapsulated within a plastic block. Typically, a thermosetting plastic,such as epoxy, is used and encapsulation is accomplished by transfermolding. Then, the encapsulated components are removed from the mold andaged so as to complete polymerization of the thermosetting plastic.

In order to avoid extrusion of the plastic encapsulating material intounwanted areas during molding, "dams" 22, as seen in FIG. 2, areincorporated by each of the leads within the lead frame. After curing,the "dams" must be removed in order to prevent a short circuit. This istypically accomplished by a punch press.

Despite the use of the "dams", some plastic, known as "flash", extrudesbetween the boundaries of the mold cavity and adheres to the lead frame.This flash must be removed from the lead frames before the package isessentially complete.

The present invention as shown in FIG. 3 provides a casing 30 for anelectrical component 32. A metal lead frame 34 has the electricalcomponent 32 electrically connected thereto. A metal base member 36 isbonded to one surface 38 of the lead frame. A plastic housing member 40is bonded to a second surface 42 of the lead frame. Adhesive material 44seals and bonds the metal base member and the plastic housing member tothe lead frame so as to enclose the electrical component 32.

As seen in FIG. 3, the terminal leads 46 and 48 project between the basemember and the housing member external to the casing 30. Although theterminal leads preferably protrude from two sides of the casing, it iswithin the scope of the present invention for the terminal leads toextend from any number of sides. There may be surface areas between thehousing member 40 and base member 36 which are in contact with eachother. Then, the adhesive provided between these surface areas wouldbond and seal them together. The housing member or premolded plastic lid40 may be shaped to form a hollow enclosure as shown so as to notinterfere with the chip 32 or electrical wires 50. However, the exactshape of the casing 30 does not form a part of the present invention andmay be any shape as desired.

The metal substrate 36 consists of a high thermal conductivity metal.The substrate is preferably formed of a high conductivity copper orcopper alloy such as CDA alloy C151. To provide better adhesion, thehigh conductivity metal substrate may be clad with a thin layer 37 ofany desired metal or alloy which bonds strongly with an adhesive. Inaccordance with this invention, it has been found that certain copperand copper base alloys provide very strong bonds with differentadhesives and thereby prevent diffusion of contaminants between theadhesive and the metal substrate. Suitable copper base alloys for use inthe present invention contain from 2 to 12% aluminum. In particular, CDAalloy C638 as disclosed in U.S. Pat. No. 3,676,292 to Pryor et al.,containing 2.5 to 3.1% aluminum, 1.5 to 2.1% silicon, and 0.25 to 0.55%cobalt is most useful in the provision of a metal in accordance withthis invention. Impurities may be present which do not prevent bondingin a desired environment. Further, it may be preferable to use CDA alloyC6381, as disclosed in U.S. Pat. No. 3,341,369 to Caule et al. AlloyC6381 is similar to alloy C638 except for the omission of cobalt. Thisomission reduces cost and entails some reduction in annealed mechanicalproperties.

The advantage of using either alloy C638 or C6381 is that they bond wellto adhesives, such as epoxies, and are, therefore, substantiallyimpervious to the diffusion of contaminants between the epoxy and themetal substrate. Also, the metal is an excellent barrier to anycontaminants passing directly through the casing. Thus, the casing ofthe present invention has reduced the potential for interdiffusion ofcontaminants since a substantial portion of the package is metal and theinterdiffusion can substantially only occur in the relatively thin layerof adhesive which seals the package and possibly through the preformedlid.

The lead frame 34 may be similar to the lead frame as shown in FIG. 2.However, the dams 22 are not required since there is no molding causingan extrusion of plastic from the mold cavity. The lead frame is formedof a material which strongly bonds to the adhesive. Preferably, thismaterial is CDA alloy C638 or C6381 as described hereinabove.

A premolded plastic lid or housing member 40 is provided to enclose thechip 32 in the casing 30. The premolded plastic lid is preferably formedof a suitable thermoplastic resin which may be particularly imperviousto diffusion of contaminants therethrough. The lid 40 may be formed inany desirable configuration so as to provide an adequate space tocontain the chip.

The present invention uses any adhesive 44, such as an epoxy, whichprovides a strong bond with any metal or alloy and with the plastic top.The specific adhesive used is determined by the constraints implied bythe specific chip and the different service environments. Some specificfactors to consider are the frequency of the electrical signal, thedielectric constant, the sensitivity of the chip to contaminants,constituent bleedout of the adhesive, and the a adhesive's permeabilityto moisture.

A number of different adhesives have been found to bond and seal thelayers together. Several examples are provided in the chart below.

    ______________________________________                                        Type        Name         Manufacturer                                         ______________________________________                                        a.  2 part room Bondmaster M688                                                                            National Adhesives                                   temperature              Co.                                                  structural                                                                    eooxy                                                                     b.  1 part therm-                                                                             Morton 4l0B  Morton Chemical Co.                                  osetting                                                                      encapsulating                                                                 epoxy                                                                     c.  thermoplastic                                                                             Philips Ryton                                                                              Philips Petroleum                                d.  encapsulating                                                                             Plaskon X7200                                                                              Plaskon Electronic                                   epoxy       Plaskon X7200LS                                                                            Materials                                        ______________________________________                                    

The sequence of manufacturing the casing of the present invention isprovided hereinbelow with certain advantages pointed out. First, thelead frame 34 is adhesively bonded to the substrate 36 using an epoxy orother adhesive material as set out hereinabove. Then, the chip may beadhesively bonded to the substrate using the type of adhesives normallyused in conventional packages described hereinabove. Since mostintegrated circuits use adhesive chip bonding, it is possible to combinethese first two assembly operations into a single sequential automatedoperation. Alternatively, the chip may be metallically bonded to thesubstrate as described above. In this case, the chip bonding operationprecedes the lead frame bonding operation in order to avoid exposure ofthe adhesive lead frame bond to the elevated temperatures which may berequired for metallic chip bonding. Next, the chip is connected to thelead frame with either lead wires or with a TAB spider, as in the caseof conventional package assembly. Finally, the premolded plastic lid isadhesively bonded to the lead frame and/or metal substrate to completethe closure of the package.

Since the package is not molded around the lead frame-chip subassembly,it is not necessary to provide a dam to minimize extrusion of theplastic from a mold cavity. It follows that it is also not necessary toremove either the dam or any flash. By eliminating these two operations,four manufacturing operations can replace the previous requirement ofsix. However, the substrates must be stamped from metal strip as aseparate preliminary operation. This may be accomplished by high speedin a blanking press. Additionally, the lids must be premolded. This alsocan be accomplished at high speed using a thermoplastic resin.

In the present technology, parts are handled in sticks consisting offrom 10 to 20 lead frames carried together in a web. After chipattachment and wire bonding, the sticks are individually assembled intocavities in a multi-cavity transfer molding device. In the proposedtechnology, handling the sticks is not required. The assembly may beautomated on a reel-to-reel basis; alternatively, individual componentsmay be magazine fed to a continuously operating automated assembly line.Of course, handling the sticks is still possible if for some specialreason this is considered desirable.

A significant advantage of the present invention resides in the abilityto dissipate heat of the package. In the proposed package, heat isconducted directly from the chip to the high conductivity metalsubstrate. It need only traverse a very thin layer of adhesive used inthe adhesive chip mounting. The impedance of this thin layer may befurther reduced by using a metal powder filled, thermally conductiveadhesive. The heat transfer path out of the package is broad andrelatively short through the high conductivity substrate. Heat may bedissipated from from the substrate 36 by convection and/or radiationfrom its outer surface 54. Alternatively, the high conductivitysubstrate may be attached to elements on a printed circuit boardexpressly designed to provide for heat dissipation. These may be eitheron the primary surface of the board, or the substrate may be fitted intoa recess in the surface of the PC board. Preferably, the substrate isbonded with solder 60 to a ground plate 62 as shown in FIG. 4.

A major source of contaminants which may enter a plastic package is thediffusion through the plastic of various contaminating gases,particularly water vapor. These contaminating gases may have a verydeleterious effect on the chip. Of course, the metallic substrate isimpervious to such diffusion and its use somewhat mitigates infiltrationof water vapor and other contaminants. Also, the plastic selected forthe lid may be relatively impervious to interdiffusion since its moldingcharacteristics are not limited to those which are compatible withmolding around a finished device as in the case of the conventionalpackages. To further inhibit interdiffusion into the package, the moldedplastic lid 68 may be modified as in FIG. 6 to provide a metal foilmembrane 70 on its inner surface to provide an impervious barrier tointerdiffusion. This metal foil may be formed of any desirable metal oralloy and attached to the lid in any conventional manner. Anotherembodiment of this concept is illustrated in FIG. 7 where the metal foilmembrane 72 is provided on the outside surface of the molded plastic lid74. In either of these embodiments, the only remaining path fordiffusion is the thin layer of adhesive which is used to bond the leadframe to the substrate and the lid.

Another embodiment of the invention provides a layer of moisture orcontaminant absorbing material 80 placed upon the internal surface ofthe lid 82. This material scavenges water vapor and other contaminantswhich may diffuse into the package. The layer may consist of materialssuch as silica gel, activated carbon or mixtures of such materials. Thematerial may be joined to the lid 82 by any conventional technique.

In addition to the advantage of better thermal dissipationcharacteristics and of higher reliability, the package of the presentinvention offers the additional advantage of eliminating significantdynamic stresses upon the wire and the delicate bond between the wireand the device and the lead frame. This differs from conventionalpackaging where the molded encapsulant flows around the device, the leadframe and the lead wires and is in contact with them. As the relativelyviscous molding compound flows around the wires, significant dynamicstresses are imposed. The present invention eliminates these problemsand thereby improves the reliability of the completed casing. Inaddition, materials present in the molding compound which may bedeleterious to the device, the bonding wire or the lead frame may reactwith these components and particularly with the chip. This results indegradation of the device which is not present in the embodiments of thepreseht invention.

The patents, patent applications, and publications set forth in thisapplication are intended to be incorporated by reference herein.

It is apparent that there has been provided in accordance with thisinvention a casing for an electrical component and a process formanufacturing the casing which satisfies the objects, means, andadvantages set forth hereinabove. While the invention has been describedin combination with the embodiments thereof, it is evident that manyalternatives, modifications, and variations will be apparent to thoseskilled in the art in light of the foregoing description. Accordingly,it is intended to embrace all such alternatives, modifications, andvariations as fall within the spirit and broad scope of the appendedclaims.

I claim:
 1. The method of forming a casing adapted to receive anelectrical component, comprising the steps of:providing a metal basemember; providing a metal leadframe having first and second oppositelydisposed surfaces; disposing the first surface of said metal leadframeadjacent said first surface of said leadframe; providing a premoldedplastic housing member adjacent the second surface of said leadframe,said premolded plastic housing member having an inner surface shaped toform a hollow enclosure, said enclosure adapted to receive saidelectrical component, said housing member further having a metal foilsubstantially over the inner surface of said housing member to form asubstantially impervious barrier to interdiffusion of contaminants intosaid enclosure through said housing member; and adhesively bonding saidmetal leadframe between said housing member and said base member forsealing said metal leadframe.
 2. The method of claim 1 further includingthe step of:electrically connecting said electrical component to saidleadframe prior to bonding said housing member and said base membertogether whereby said electrical component is sealed within saidenclosure.
 3. The method of claim 2 including the step of selecting saidbase member and said leadframe from a copper or copper base member. 4.The method of claim 3 including the step of selecting copper alloyconsisting essentially of 2 to 12% aluminum and the balance essentiallycopper.
 5. The method of claim 4 including the step of selecting copperalloy consisting essentially of 2.5 to 3.1% aluminum, 1.5 to 2.1%silicon and the balance essentially copper.
 6. The method of claim 5including the step of cladding said base member with a high conductivitymetal or alloy layer.
 7. The method of claim 6 including the step ofdisposing a contaminant absorbing material within the hollow enclosureof said premolded plastic member.